Depending on the physical structure, the capacitance between a target electrode and a sense electrode varies inversely proportional to their relative distance, inversely proportional to their relative distance squared, or some functional dependence between inverse and inverse quadratic depending on the nature of the physical structure. The maximum distance at which a capacitance sensor system can detect target conductors in the vicinity of its sensor area is dependent on the minimum capacitance the system can resolve. If the capacitance of the sensor electrode relative to its ambient environment, its reference capacitance, is large compared to the capacitance between the target electrode and the sensor electrode, the capacitance sensor system sensitivity is significantly degraded. The size of the sensor electrode is dictated by the size of fingerprint artifacts, which is typically about the size of a 100 micrometer square. Being part of an integrated circuit whose vertical dimensions are small compared to 100 micrometers, the sensor electrode itself has significant capacitance to the substrate on which it mechanically rests.
For use in measuring the positions of fingerprint artifacts, a sensor array composed of an array of sensor electrodes was disclosed by Knapp in U.S. Pat. No. 5,325,442. Each sense electrode is connected through a passive switch to array wiring that is the length of the array. The array wiring is connected to a charge sensing circuit to determine the capacitance. The capacitance sensitivity is degraded by the array wiring as the effective reference capacitance on each sensor electrode increased. Additionally, semiconductor switches are introduced into the sensor area where they may be damaged by mechanical contact with the target electrode, or may leak due to photocurrent when the sensor is operated in a high-light-level environment. Additional coatings may be applied to the sensor surface to reduce the sensor's susceptibility to damage, but at an increase in the sensor to target electrode distance.
In U.S. Pat. No. 6,049,620, Dickinson et al. disclose a technique to measure the capacitance at each sensor electrode using a low value current source and additional active circuitry. A signal proportional to the total sensor capacitance is switched onto the array wiring after being passed through a source follower thereby isolating the wiring capacitance from the sensor electrode. With this technique the reference capacitance value is dominated by the sensor electrode capacitance and the capacitance of the circuitry connected to the sensor electrode itself.
In U.S. Pat. No. 6,097,195, Ackland et al. disclose a method to reduce the sensor electrode capacitance by introducing a shield electrode between the sensor electrode and the grounded physical support structure. This reference capacitance cancellation technique is applied individually to each sensor electrode, resulting in a significant reduction in the reference capacitance and a proportional increase in the sensor capacitance sensitivity. A unity gain amplifier is connected between the sensor electrode and the shield electrode with one amplifier used per sensor electrode. The increase in sensor complexity increases the sensor cost and the risk of damage from the target structures.
Other capacitive sensor systems have been described which add circuitry to the sensor array as well as additional sensor electrodes. In U.S. Pat. No. 6,114,862, Tartagni et al. disclose a capacitance sensor with active circuitry and special electrode configurations designed to improve the capacitive sensor sensitivity. The capacitance sensor use two electrodes at the sensor surface connected to opposite ends of an amplifier. Target structures such as fingers near either electrode modify the capacitance between electrodes. Both electrodes occupy the sensor surface, which increases the sensor cell size and cost.
RELEVANT LITERATUREU.S. Patent DocumentsU.S. Pat. No.DateInventorU.S. Class4,210,899July 1980Swonger, et al.340/146.3 E4,435,056October 1982Tsikos340/146.3 E4,429,413January 1984Edwards382/44,526,043July 1985Boie, et al.73/862.045,195145March 1993Backus, et al.382/45,325,442June 1994Knapp382/45,434,446July 1995Hilton, et al.257/5035,778,089July 1998Borza382/1245,828,773October 1998Setlak, et al.382/1265,978,496November 1999Harkin382/1246,049,620November 2000Dickinson, et al.382/1246,055,324April 2000Fujieda382/1246,061,464May 2000Leger382/1246,097,195August 2000Ackland, et al.324/7196,114,862September 2000Tartagni, et al.324/6626,289,114September 2001Mainguet324/1246,317,508November 2001Kramer, et al.382/1246,365,888April 2002Von Basse, et al.250/208.1